The invention relates to a device for measuring the volume of a gas, comprisng a cylinder and a piston movably mounted in said cylinder, a first membrane for sealingly connecting said piston with the inner wall of said cylinder, said cylinder, said piston and said first membrane determining a measuring space in which a measuring conduit for the gas to be measured debouches.
Such a device is known from Dutch patent application No. 67.07816. In order to make a measurement as accurate as possible with a device of this type, the piston should be movable in the cylinder as easy as possible, i.e. at a pressure increase as low as possible. To this end the membrane should unroll smoothly at the movement of the piston. In order to fulfil this requirement a thin film of flexible material is chosen as membrane at the known device, wherein for a correct unrolling the space between the piston and the cylinder inner wall must be chosen small. This results in the disadvantage of a strong bend in the membrane and thereby in frictional losses.
During use of the device when sucking or blowing in the measuring space, the pressure in the measuring space may become negative or positive with respect to the ambient pressure. Depending of the position of the membrane with respect to the measuring space, the membrane will come off from the piston or the cylinder inner wall and drag with itself. This results not only in further frictional losses but moreover an unallowable measuring error will occur as a movement of the membrane transveers to the longitudinal axis of the cylinder causes a volume variation of the measuring space without a movement of the piston so that this volume variation is not determined.
It is noted that a cylinder piston assembly is known, in which the piston is sealingly connected with the cylinder through a membrane and in which a transverse movement of the membrane is obviated by generating a pressure difference across the membrane. However in this case a restriction is provided in the discharge conduit connected to the space determined by the piston, the cylinder and the membrane. The use of such a restriction is not allowable at the device according to the invention as an accurate gas volume measurement can not be made anymore.